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The short time available for injection and mixing in high-speed engines requires an accurate modeling of the fuel related processes to obtain a valuable in-cylinder charge description, and then a good combustion performance prediction. An advanced version of the KMB code of IFP has been used to compute a racing engine. It includes a fitted on experiments spray model, a comprehensive wall-film model, the AKTIM ignition and ECFM combustion models. A major difficulty was the necessity to compute numerous cycles before reaching a cycle-independent solution. A procedure has been defined to minimize calculation time. Another difficulty was the high concentration of liquid in some zones, which requested a careful meshing. Effects such as the influence of the strong acoustic waves on the spray dynamic, the wall wetting effects on the engine time response, injector position on fuel distribution in the cylinder, charge homogeneity on the combustion process have been investigated.

Currently, the development of higher specific output and higher efficiency S.I. engines requires better control and knowledge of knock mechanisms. As it is not easily possible to instrument an engine to determine the beginning of fuel auto-ignition, knock modeling by means of 3D CFD simulation, can be a powerful tool to understand and try to avoid this phenomenon [1, 2, 3]. The objectives of the work described in this paper are to develop and validate a simple model of auto-ignition. This model, developed at IFP, is implemented in the 3D CFD code KMB [4, 5]. It is based on an AnB model [6, 7] which creates a ‘precursor’ species transported with the flow in the combustion chamber. When its concentration reaches a limiting value, the auto-ignition phenomenon occurs.

Gasoline Direct Injection (GDI) is today more regarded as a suitable technology for relatively high displacement engines. The literature shows that the R&D effort on GDI engines is generally made for bores larger than 80 mm. But because GDI appears to be the most relevant way to improve fuel efficiency of S.I. engines, it should also be considered for small bore engines (bore below 75 mm). Nevertheless, locating an injector in already congested cylinder heads, with ultra lean stratified combustion capability while maintaining high engine specific power and proper cylinder head cooling is a real challenge. For such an engine, IFP “narrow spacing” proposal is a 3-valve per cylinder layout or NSDI-3 concept, with a spark-plug-close-to-the-injector design and a suitable piston to confine the fuel spray within the vicinity of the ignition location. This paper describes stage by stage the prototype engine realization using this novel concept.